This invention relates to a semiconductor light emitting element, especially one such as LED (light emitting diode) or semiconductor laser diode, for example, made of nitride compound semiconductors, and having an active layer containing indium (In).
Nitride compound semiconductors ensure efficient luminous recombination because of their optical transition being of a direct transition type, and their development is under progress toward the use as materials of semiconductor lasers, high-luminance LEDs and other high-efficiency light emitting elements.
In the present application, the term xe2x80x9cnitride compound semiconductorxe2x80x9d pertains to any III-V compound semiconductor expressed by BzAlyInxGa(1xe2x88x92xxe2x88x92yxe2x88x92z)N (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61), and group V elements are construed to also involve mixed crystals containing phosphorus (P) and/or arsenic (As) in addition to N.
Indium gallium nitride (InxGa1xe2x88x92xN), one of mixed crystals of nitride compound semiconductors, can be changed in band gap energy from 3.42 eV of GaN to 2 eV of InN by adjusting the mole fraction X of In, and can be used as an active layer of a visible light emitting element.
So-called xe2x80x9cdouble-heterostructurexe2x80x9d is often used in semiconductor lasers, light emitting diodes or other semiconductor light emitting elements. This is a structure in which an active layer (or emission layer) having an energy band gap corresponding to a predetermined emission wavelength is sandwiched by cladding layers with a larger band gap. Usable as the material of the emission layer of the semiconductor light emitting element having a double-heterostructure is indium gallium nitride (InxGa1xe2x88x92xN) which is one of mixed crystals of nitride compound semiconductors. Indium gallium nitride can be changed in band gap energy from 3.42 eV of GaN to 2 eV of InN by adjusting the mole fraction X of In, and can be used as the active layer of a visible light emitting element.
However, existing semiconductor light emitting elements using indium gallium nitride as their emission layers involved the problem that the lifetime may be insufficient.
FIG. 8 is a graph showing a result of a reliability test of an existing semiconductor light emitting element in changes with time of optical outputs of a blue light emitting diode using as its active layer a mixed crystal, i.e., indium gallium nitride (InxGa1xe2x88x92xN) in which the mole fraction X of indium is 20%. The emission wavelength is about 450 nm, the operative current of the light emitting diode is 20 mA, and its operative temperature is the room temperature. It is known from FIG. 7 the optical output of the light emitting diode suddenly begins deterioration approximately after 1000 hours, and deteriorates to approximately a half in 10000 hours.
This decrease in optical output causes problems in various fields of its application. For example, in the case where a blue light emitting element using indium gallium nitride as its active layer is employed in a traffic signal, a decrease in emission luminance will seriously degrades the visibility. In the case where it is employed as a light emitting element for reading and writing data in an optical disk system, it will become difficult to write data, and read errors will increase. When it is employed in a multi-color display or full-color display, its deterioration with time will appear as irregular colors.
It is therefore an object of the invention to provide a semiconductor light emitting element having a sufficient life time and selective in emission wavelength from a wide ranges of wavelengths including blue, green and orange as well.
The Inventors found through his own researches that various characteristics can be improved in a light emitting element using a nitride compound semiconductor containing In (indium) as its active layer by adding at least one of Al (aluminum) and B (boron). More particularly, the thermal resistance of the crystal can be improved, and the reliability of the element characteristics can be improved remarkably.
The above-mentioned feature of the invention is very unique. When In is added to GaN, the band gap energy becomes smaller than 3.42 eV which corresponds to the band gap energy of GaN. Naturally, the emission wavelength becomes longer than 364 nm which corresponds to the wavelength of GaN. In contrast, when Al or B is added to GaN, the band gap energy becomes larger than 3.42 eV, and the emission wavelength becomes shorter than 364 nm.
Therefore, only In or Al or B has been added to GaN depending upon the required emission wavelength heretofore. Only indium has been conventionally added to GaN to form the emission layer for the emission wavelength range longer than 364 nm.
In contrast to this, the Inventors found that a light emitting element can be remarkably improved in reliability by adding both In and Al or B while keeping the emission wavelength longer than 364 nm.
InGaN conventionally used as an active layer inevitably caused deterioration of element characteristics because of insufficient thermal crystalline stability. The invention is characterized in improving the reliability of the element by incorporating a Al-based and/or B-based mixed crystals to InGaN to achieve a relatively high thermal stability. When an AlInGaN, BInGaN or BAlInGaN semiconductor is used as the active layer, light in the range from the visible band to ultraviolet wavelengths can be obtained by adjusting the energy gap relying on concentrations of In and Al or B.
Structural features of the semiconductor light emitting element according to the invention lie in: an active layer being made of a nitride compound semiconductor containing at least one of Al and B, In, Ga and N and having an energy gap as small as 3.42 eV; and emission being effected by band-to-band recombination.
According to the invention, characteristics of the semiconductor light emitting element can be stabilized by adding Al (aluminum) and/or B (boron) to InGaN forming the active layer. That is, the semiconductor light emitting element according to the invention does not degrades in optical output even after 10000 hours, and its lifetime is remarkably improved as compared with an existing semiconductor element for emitting blue light in the same wavelength range. In this manner, according to the invention, the life time of the semiconductor light emitting element is improved remarkably by increasing the mole fraction of indium in a conventionally used ternary mixed crystal and adding aluminum or boron to indium gallium nitride thereto so as to adjust the band gap.
The prominent features of the invention are against the existing common view, and have been obtained by Inventors"" own researches.
Moreover, according to the invention, the difference in lattice constant can be reduced to improve the reliability of the light emitting element furthermore by using GaN as the material of layers adjacent to the active layer. That is, when cladding layers, for example, adjacent to the active layer are made of GaN, the difference of its lattice constant from the active layer having a band gap corresponding to one from blue to green can be reduced to further improve the lifetime of the light emitting element.
Emission characteristics can be improved also by doping silicon to the light emitting layer. That is, the practical concentration of silicon used in the invention ranges from 1xc3x971017cmxe2x88x923 to 1xc3x971021cmxe2x88x923, and the light emitting element with a silicon concentration in this range exhibited no deterioration. Thus, it has been confirmed that the light emitting element according to the invention be highly reliable even when the luminous intensity is increased by doping silicon in this range of concentration.
As summarized above, the invention successfully realizes a semiconductor light emitting element with high reliability and high performance, and its industrial merit is great.